1. Field of Invention
This invention relates generally to a two-wire telemetering system in which a direct-current signal generated by a transmitting device is conveyed over a two-wire line to a receiving station having a DC power supply whose output is supplied to the transmitting device over the same line to provide operating power therefor, and more particularly to a system of this type capable of supplying a relatively large amount of power to the transmitting device.
2. Status of Prior Art
A two-wire telemetering system is useful in an industrial process control loop in which a value sensed at a transmitting device by a thermocouple or other sensor of the process variable being metered is converted into a direct current signal that is conveyed over a two-wire line to a remote receiving station for operating indicators, recorders or controllers in the process control loop.
One important advantage of a two-wire telemetering system of this type is that the same wires serve not only to convey the current signal from the transmitting device to the station but also to conduct operating power from the receiving station to the transmitting device. The dual use of the power supply line as the signal line obviates the need for extra wires in remote control measurement. Also, a current output minimizes susceptibility to voltage noise spikes and eliminates line drop problems.
For such a telemetering system, American National Standard ANSI-MC 12.1-1975 and ISA-S 50.1, "Compatibility of Analog Signals for Electronic Industrial Process Instruments" specify that the standard output signal (of a transmitting device) shall be of a constant current nature having a range of 4 mA to 20 mAdc [Section 3.2 of the Standard], and that the standard voltage signal (of the receiver) shall be 1 volt to 5 volt dc [Section 3.3.2 of the Standard]. These standards are generally accepted and practiced by the industrial process control industry. Hence in most industrial process control instruments, 4 mAdc is assigned to the direct-current I.sub.0 at 0% signal and 20 mAdc to the direct-current I.sub.100 at 100% signal.
Thus the direct-current signal generated by the transmitting device lies in a 0% to 100% signal range in which the ratio of current I.sub.0 (=4 mA) at 0% signal to current I.sub.100 (=20mA) at 100% signal is equal to 1/5 or 0.2. At the receiving station, this current signal flows through a terminal resistor to produce a voltage signal thereacross of 1 to 5 volts whose ratio of voltage V.sub.0 at 0% to the voltage V.sub.100 at 100% is equal to 1/5 or 0.2.
It has been found that known telemetering systems of this type fail in some instances to supply adequate operating power for transmitting devices. For example, if the transmitting device is a differential-pressure (D-P) transducer operating in conjunction with a square root extractor, the power demand of the D-P transducer and the associated square root extractor are not satisfied at low input levels when the device operates in the usual 4 to 20 mAdc range. And if one wishes to include a microprocessor in a transmitting device, because of the existing constraints in power availability, this may not be possible.
By reason of this power limitation, it becomes necessary in many instances to operate the telemetering system in a four-wire configuration rather than with a two-wire line. The additional wires are required to convey adequate operating power to the transmitting device, thereby losing the important benefits of a two-wire system.